Yellow laser sources are of interest for various applications. The 573-580 nm band is attractive for ophthalmology and dermatology applications, and 589 nm is a wavelength of interest for laser guide star applications. Presently, coherent radiation in this wavelength band is usually provided by bulky and expensive laser sources, such as dye lasers and copper-vapor lasers. Naturally, it would be highly desirable to provide an efficient diode-pumped solid-state laser alternative. Such a solid-state source would include a device that performs nonlinear frequency conversion, since yellow wavelengths are difficult to generate directly from solid-state laser media.
Attempts have been made along these lines. For example, Pennington et al. consider frequency summing the outputs of two fiber lasers (operating at 1583 nm and 938 nm) to provide a yellow source in “Compact fiber laser approach to 589 nm laser guide stars” presented at the Conference for Lasers and Electro-Optics in 2004. Frequency doubling of a Raman-shifted fiber laser has also been considered (e.g., by Sharma in Reviews of Laser Engineering vol. 33, p. 130, 2005). In these examples, special measures are employed (e.g., Raman shifting or sum frequency generation) to overcome the difficulties posed by the lack of efficient high-power solid-state laser sources suitable for direct doubling into the yellow. However, such special measures introduce undesirable complications, and it would be preferable to obtain the desired yellow radiation more simply by frequency doubling.
Direct frequency doubling to the yellow has been demonstrated by frequency doubling a Yb:YAG microchip laser to 565 nm (Burns et al., Optics Communications vol. 207, p. 315, 2002). However, low output power (<1 mW) and low efficiency (<0.1%) were reported in this work. It has proved difficult in practice to provide laser sources more suitable for direct frequency doubling to the yellow. To appreciate these difficulties, it is helpful to consider a specific case.
Yb-doped silica is an attractive gain media for the 1-μm band because of its wide gain bandwidth and high quantum efficiency when pumped at 980 nm. Although the gain bandwidth of Yb-doped silica extends to around 1200 nm, it has been difficult to make fiber amplifiers and lasers operating at wavelengths greater than 1120 nm (which could be frequency doubled to provide yellow radiation). Two reasons for this difficulty are the low emission cross section at these wavelengths, and amplified spontaneous emission (ASE) at shorter wavelengths (especially in the vicinity of 1030 nm, where the gain is much higher), which depletes the Yb population inversion and thus reduces the gain available at longer wavelengths.
A further complication is photodarkening (loss due to optical damage to the fiber), which extends to 1150 nm in Yb-doped silica. Photodarkening is a process by which the high intensity of pump and/or signal light traveling through the gain medium (e.g., a fiber) induces a loss in the gain medium, usually via the creation of color centers. This mechanism has been observed and studied in many laser materials, including recently in Yb-doped silica fibers, as reported by J. J. Koponen et al., “Photodarkening in Ytterbium-doped Silica Fibers”, Proc. SPIE vol. 5990, paper c3, 2005. In Yb-doped fibers operated at longer wavelengths, even a small additional loss from photodarkening can prevent lasing completely or severely degrade efficiency, due to the low gain at long wavelengths. Mitigation of photodarkening is considered in U.S. Pat. No. 6,154,598, where photodarkening due to unintentionally incorporated impurities is reduced by adding suitable rare-earth co-dopants to effectively shunt the upconversion process responsible for photodarkening. However, this technique may not be applicable if photodarkening results from the same dopant that provides the gain (as opposed to resulting from an unintentional impurity).
Another practical issue that can arise is excessive noise and/or self-pulsing of the laser. U.S. Pat. No. 5,953,353 considers an approach for mitigating this problem by increasing the cavity length of the fiber laser, typically by including a length of undoped standard single-mode fiber into the laser cavity such that 30% or more of the cavity length is undoped fiber.
Since there is an unmet need in the art for an efficient solid-state laser source suitable for direct frequency doubling to the yellow, it would be an advance in the art to provide such a source. It would also be an advance in the art to mitigate photodarkening in fiber lasers.